The present invention relates to an obstruction detection device, in particular to an infrared intruder detection system.
Passive infrared detection systems are widely used in intruder detection systems. Their underlying principle is to detect far infrared radiation (wavelength greater than 10 μm). This radiation is emitted by any warm body, e.g. by a human, vehicle. A respective infrared sensor is commonly placed behind an entrance window to protect the sensor against the environment.
At daytime, most intruder detection systems are deactivated. An intruder can now manipulate the passive infrared detectors such that they remain inactive permanently. One kind of manipulation is to disguise the entrance window by a spray or liquid, which is opaque for far infrared radiation, but transparent for visual or near infrared radiation. Maintenance staff of the intruder detection system cannot see this spray and detect the manipulation of the passive infrared detector just by a glance.
According to EP 0 660 284 A1 a near infrared emitter is placed outside of an entrance window of a passive infrared detector. The emission angle of the emitter is very broad, and a part of the near infrared light will be detected by a near infrared sensor placed behind the entrance window. A spray applied to the entrance window, that is opaque for near infrared radiation will be easily detected. A spray transmittive for near infrared radiation instead can be used to sabotage a passive infrared detector.
EP 0 772 171 A1 describes a sabotage detection system, which uses a diffractive surface. Light from a light source is focussed to a detector by the diffractive surface. A spray applied to the structured diffractive surface changes the diffractive pattern and the focus point. This leads to a change in the intensity of light detected by the detector. Unfortunately, it is difficult to manufacture the complex diffractive surface in cheap and widely used synthetic materials.
U.S. Pat. No. 5,499,016 and EP 0 817 148 A1 propose to use an infrared emitter and a detector both arranged at the outer side of the entrance window. The infrared radiation of the emitter is scattered on the surface and in volume of the entrance window. The volume scattering is dominant. The reflected parts are detected by the near infrared detector. A spray applied to the surface of the entrance window partly changes the reflective properties of the entrance windows and thus the intensity detected by the near infrared detector. A spray applied to the entrance window will basically form a smooth film. The differences of the surface properties of the entrance window and the liquid contribute to a change of the intensity of light scattered to the detector. This change, however, is very small. The dominate part of the light scattered by the volume is not affected by the liquid and remains unchanged. Thus highly sensitive detectors are necessary in order to measure the small change. The mechanical set-up of EP 0 817 148 A1 uses light guides for emitting and detecting light to and from the entrance window, respectively. A grazing incidence of the light is achieved, which increases the sensitivity on a spray applied to the entrance window, but on the expense of a complex mechanical light guide structure.